The conventional mold fabrication industry usually uses CNC machine tools, copy engraving machines or electric spark machining to do fabrication directly on molding steel. Due to limitations of the tools and precision of the machine tools, conventional machining cannot fabricate molds of miniature geometric profiles, special optical characteristics and high levels of precision, such as the miniature molds for fabricating micro devices. Such molds have to be fabricated by electroforming.
Electroforming is an electroplating technique to produce a thick layer of metal shell mold on a mandrel. The metal shell is peeled off the mandrel to become a duplicate of the mandrel, and it is finished by machining and trimming. It can be used to produce precise and complicated molds in large quantity that are difficult to fabricate by conventional machining or require too much manpower. The duplicate may reach the precision of 2.5 μm and the surface resolution can reach 0.02˜0.05 μm. Hence this molding technique is desirable for fabricating molds used in production of CDs, reflection sheets of car lights, light guide plates of liquid crystal displays and the like.
However, in conventional electroforming techniques, the depositing speed of electroforming is very slow, about 0.5-1 mm/day. For instance, to form a mold insert by electroforming for a thickness of 30 mm, submerged electroforming time could be six to eight weeks. The slow depositing speed of the electroforming process is a technical bottleneck awaiting resolution.
On the other hand, metal spraying is a physical metal depositing technique that has a high depositing speed (about 20-180 kg/hr). It can be used to brace and thicken the electroformed mold insert, greatly reduce the submerged electroforming time, and fabricate the mold at a faster speed.
However, the metal spray thickened layer and the electroformed layer are bonded by mechanical keys. The metal spray thickened layer is formed by stacking hundreds of metal films. The metal spray film fabricated by the present metal spray equipment has tension stress. This tension stress increases as the thickness of the metal spray layer increases. When the tension stress of the metal spray thickened layer is greater than the key bonding strength of the metal spray thickened layer and the bottom material, the metal spray thickened layer can easily peel off. The poor bonding strength of the metal spray layer is the main drawback of the metal spray thickening process. Moreover, thermal effects during the metal spray process can also cause deformation of the electroformed layer, affecting the strength of the metal spray layer and resulting in soldering defects. These are the disadvantages of the metal spray technique.